Friday, November 27, 2015

Hey? Nobody has asked about the Crystal Filters that are being used with the Simpleceiver Project!

Addendum: 11/29/2015 Another on the air video of the Simpleceiver Addendum #2: A view of the completed Simpleceiver "al fresco"Addendum #3: 12/01/2015 Data for a 20M RF AmpAddendum #4: Signal Output Data for the AD9850Addendum #5: Band Pass Filter Data for 20 Meters

The following is the schematic for the RF Amplifier stage. Please note about the Resistor "R" and how that is made.

Addendum #3: To use this RF Amp on 20 Meters (Query from WA7RHG) simply make L1 = 8 Turns on the FT-37-43 core and make C2 = 10NF.

Addendum #4:Based on anearlier input regarding why I used the AD9850 straight into the SBL-1 without a "booster amp". I guess the simple answer is I hooked it up and it worked --so keeping thing simple. But to get full advantage of the SBL-1 it probably would be a good idea for a booster amp. Now when you measure the output of the AD9850 + Booster Amp make that measurement with a 50 Ohm load and a scope. Do not do it connected to the SBL-1.

Here are three measurements:

With the AD9850 terminated into 50 Ohms V= 324 MV PTP

With the AD9850 No Load V = 800 MV PTP

With the AD9850 connected to the SBL-1 V = 440 MV PTP

The SBL-1 is a 7 dBM device and likes to see 1.414 Volts PTP and a homebrew DBM most likely will want to see 2 Volts PTP (or 10 dBm). So a homebrew DBM will need the booster amp!

Addendum #5: Band Pass Filter Data for 20 Meters. It appears there is interest in putting the Simpleceiver on 20 Meters so here is the Band Pass Filter data. Please note if you are using the AD9850 you will need USB and so you need to take the LO plus the Offset to work 20M. Thus the VFO has to be in the 2.10 MHz range for 14.2 MHz. [NOTE in Part 17 you will see why this is not a good choice for a LO Frequency.]

The Crystal Filter

I guess this is a significant input to me as I have had no inquiries about the crystal filters being used with the radio. But just in case anyone was wondering here is some preliminary information to get you started.

First it is important to start by having three or four crystals (depending on which filter you build) to have the total frequency difference be no more than 50 Hertz across the units. To make that clear when you measure the frequencies the total difference from high to low of any of the crystals should be no more than 50 Hertz!!!! Typically I buy a dozen crystals and from that batch will find that you can usually squeeze two filters with crystals that match that hurdle. Avoid buying four crystals and simply plugging them into the circuit. If by chance you would do this and the four crystal you purchased meet that criteria -- then stop wasting your time on homebrew projects and buy a batch of lottery tickets for you are one lucky dude!

This now gets to the problem of how to measure the frequencies to within 50 Hz. I have found that many hams new to homebrewing really lack some basic test equipment and there are few alternate paths beyond such a situation. A homebrewer needs something more than a rusty screwdriver, a beat up electric drill and analog VOM that is 10% accurate.

Eventually the serious homebrewer needs an O scope, a stable RF signal source , a frequency counter and a Digital Volt Meter (DVM). Most of the modern Digital Storage Oscilloscopes have a built in frequency counter. Thus one way of measuring each crystal would be to build a test oscillator and measure the output from the oscillator and simply read the frequency on the scope. Another would be to have a frequency counter. (I happen to have both.) But something I have been recently using is my SDR Softrock transceiver along with the Power SDR software.

Typically with the Softrock, I fire up the oscillator and have a "sniffer loop" (short chunk of wire connected to the SDR antenna port) from the Softrock brought near the output of the oscillator and simply read the frequency from the software display. The desirability of this approach is that I set the Power SDR parameters to CW with the narrowest filter and then look for a peak reading on the display. Then I can note down the frequency.

But that only finds the three or four crystals that are close in frequency thus something more substantial is required to build a high quality crystal filter.There is a documented way to do this and I refer you to the following link Crystal Filter Construction from WA5BDU. Building a high quality filter can be done but must not be done in a haphazard manner. This tutorial is quite excellent and provides the formulas, equations and theory for filter construction.

Typically after I find the four crystals I sometimes think about purchasing a commercial filter. There is no reason that a 9.0 MHz commercial filter could not be installed in this circuit by modifying the 12 MHz amps to work on 9.0 MHz. With LT Spice that is not a difficult task. INRAD sells a 9.0 MHz a Four Pole filter with a Zin/out of 200 Ohms. (Model 315). 9.0 MHz filters are available also from the GQRP Club, Z in/out = 500 Ohmz. The beauty of the Arduino driving the AD9850 is that should you change the filter frequency, a few lines of code changes and you are there. The problem is somewhat more difficult using the LC VFO or a VXO. RF from the these LO's would have to be in the 2 or 16 MHz range for the 9.0 MHz IF. You get the idea.

Now for a not so high quality, not rigorously calculated, and not formally approved by the EMRFD and BITX reflectors, I usually skip the tutorial and use some values that seem to work for me. Here is exactly what I did here. I assumed a Z in/out of 150 Ohms. My listening tests have shown that the values are not too far off. But then again my goals are a bit different as I am trying to define a template for a complete project and taking a shortcut on the filter may not be the best practice; but it does get the radio to a least the point of working.

But I encourage the readers to do the rigorous calculations so you can say I know how to do it and have done it! A rigorously designed filter will undoubtedly perform better and will satisfy the need to have everything precise and tidy!

Once you have the filter built and before soldering in the circuit it would really be a good idea to test your filter. (It will be impossible to test using only the rusty screwdriver, a beat up electric drill and the 10% accurate analog VOM. Thus you will need something more. )

One of the test procedures is to terminate the filter with a 150 Ohm resistor and with the AD9850 programmed to be a signal generator as the input and placing the scope across the 150 Ohm resistor "sweep" the filter at every 50 Hz starting at 5 KHz above and extending to 5 kHz below the center frequency. Record the amplitude points and then make a plot of the points versus the frequency and you will get a shape of the filter.

If you use my method, it will be a bit ugly (maybe more than a bit ugly) but if you use WA5BDU's method you should see plots similar to what is shown in his tutorial. Alternatively you can use the AD9850 and if you have a SDR receiver repeat the same procedure and use the S Meter numbers for the plot data.

Or if you have one of those snazzy SNA's (Scalar Network Analyzer) you can run an automated test and see the plot

I want to repeat my "good enough" filters will work BUT you should do the rigorous approach as outlined in the link. Once you do that then there will never be a question of how good is your filter.

The final chapter for this part of the project will cover the receiver RF amplifier stage which is very similar to the IF amplifiers stages previously covered.

Since yesterday was Thanksgiving here in the USA, I am still in a partial eating too much Turkey coma.

Monday, November 23, 2015

We are nearly finished with the heterodyne receiver portion of the project! Hang in there.

Today I finished what I call the main board which is comprised of the Mixer stage (SBL-1) followed by the 1st IF Amplifier which is a 12 MHz design consisting of two J310 JFETS configured as a Dual Gate MOSFET. The board also contains the new 4 pole Crystal Filter followed by the 2nd IF AMP stage which is similar to the 1st IF Amp. Below is a photo of that board. I have tried to label the various sub-circuits so you can see the layout. The "squares" are 2/10 inch. The W1REX MePads would work ideal for this board.

Next here is a You Tube Video of the 3rd Generation. Close your mouth --yes that is a homebrew Simpleceiver that you can build!

The last stage to be built is the RF Amplifier stage. This stage is very similar to the IF amp stages in design and given with what I have seen with the IF Amp stage should work quite well.

Given we are moving into the Holiday Season work will slow down a bit on the Transmitter stages but I will update the blog every so often with any late information I may have.

Saturday, November 21, 2015

More of the Bits & Pieces

This will be a short posting to provide more information and give rise to other possible configurations. Previously we mentioned about the hard core homebrewer's who did not consider this a homebrew project unless it was totally homebrew. To that end, today, I hooked a 5 MHz LC VFO I had that was left over from a prior project wherein it was replaced with a Si5351. In further response to the hard core group, in the future I will present a design for a crystal switched VXO so that segments of the 40 Meter band can be covered using the VXO. But for now here is the LC VFO video. Note how quiet the receiver is between stations and also note I prototyped a 4 pole crystal filter that is "haywired" into the circuit.

In the most recent SolderSmoke Podcast #182 (11/20/2015) I mentioned to my friend and host Bill N2CQR about the power of the LT Spice simulations that were being used with this project and now I would like to expand a bit more on what was said.

In retrospect I am guilty of just trying things and then at times are a bit surprised when an expected performance level is on the short side. Below is the 12 MHz IF amplifier circuits that will be used in conjunction with the new 4 pole Crystal Filter. The current configuration is set to use 12 Volts to power the circuit. But I wanted to explore what happened at lower voltage levels. So OK I was toying with a spin off project called the Cellceiver. I have a big box of defunct mobile phones and in that box are a large number of still good rechargeable batteries. So the idea was to build a Simpleceiver powered from 3.8 Volts. Also important is the situation where you don't have +12 VDC and you grab a 9 VDC transistor radio battery thinking "this will work OK". Well maybe not. [Note in the schematic below the top margin got cut off but the top end of 68K gets connected to the +12VDC Rail.]

Now what happens if you change the Supply voltage [Going from +12 VDC down to + 5 VDC]?

This is amazing! As you drop the supply voltage from 12 VDC to 5 VDC, the gain drops by 2/3 from 18 dB to about 6.5 dB which is a dramatic reduction. Dropping to 9 VDC drops the gain by 4 dB thus if the circuit calls for 12 VDC then that is what it needs!

Take a look at the tank circuit and note that there is a capacitive voltage divider, C1 and C2. The equivalent series capacitance across the tank is about 23.5 PF. If you were to connect a 23.5 PF cap in series with a 10 NF that essentially is a 23.5 Capacitor. Run the simulation and look at the output level. Try putting the 23.5 PF cap on top and the 10 NF on the bottom and run the simulation. Then reverse the two with the 23.5 PF cap on top and the 10 NF on the bottom. Note the gain readings. You can draw some very important conclusions from this --which I will leave to the reader. Hint: Size and order matters!

Thursday, November 19, 2015

Second Generation Simpleceiver

This post is to let you hear how the Second Generation of the Simpleceiver sounds in operation. What I have done is to mate the front end of the 1st generation Simpleceiver consisting of the RF Amp Stage, homebrew Double Balanced Mixer, Post Mixer Amp and three pole Crystal Filter. Using this part it was "haywired" to the new J310 Product Detector, J310 BFO and the NE5534 + LM380 Audio Amp. Ahead of the 1st generation board sandwiched in between the RF Amp and the homebrew DBM, I installed the new Band Pass Filter. Take a listen how quiet it is between stations.

I did make a modification of the interface between the J310 BFO and the J310's that are connected in Cascode. On Gate #2, I have added a 22K resistor to ground and connecting the two modules is a 100 NF cap. A revised schematic is shown below as well as the plot of the output. These changes were made to accomplish two things: 1) the BFO loading is improved and the BFO now reliably oscillates every time the power is applied and 2) the overall gain of the Product Detector is improved. I found a condition that you need to tweak the BFO trimmer cap so that the signal was placed properly on the slope of the 3 pole Crystal Filter. But once you powered the circuit down there were cases where upon application of power the BFO Oscillator would not oscillate. This now has been cured.

Special Note: In the hardware configuration a Broad Band Matching Transformer is connected to Gate #1. The primary is 3 Turns and the secondary is 20 Turns on a FT-37-43 Ferrite Core. The 3 Turn winding is the input and the 20 Turn winding is connected directly to Gate #1. If we do the math the Z in is 50 Ohms and the input to Gate #1 is forced to 2.2K. 2.2K/50 = 1:44. The match is based on the turns ratio squared. 3^2 =9 and 20^2 = 400. 400/9 = 44.44. This this is a close match.

Wednesday, November 18, 2015

More Bits and Pieces

In our last post we described the Band Pass Filter simulation with LT Spice and the measurement of the filter performance. There is no one more astonished than me that the O Scope plots seem to track very well what was shown in the simulation plots. Needless to say it is a lot more productive to spend time simulating the hardware and eventually honing the design to a point where one is satisfied and to only have to solder in the components --one time! I also want to highlight the use of the older publication SSDRA by W7ZOI in conjunction with the LT Spice.

By way of review we have built the Audio Amp Stage, the Product Detector, the LO and the Band Pass Filter. This has resulted in a Direct Conversion Receiver. All of these pieces will be used in the Superhet version of the receiver albeit with a modification of the LO frequency range and the addition of new modules. Items to be added include: Beat Frequency Oscillator, Intermediate Frequency Amplifier stages, a Crystal Filter, a DBM Mixer stage and an RF amplifier stage. For those would like to follow along our block diagram is as shown below. The darkened blocks are those that were built for the Direct Conversion Receiver and those in white will be built for the Suprhet version.

Noteworthy, as this came up as a question, virtually all interfaces are at 50 Ohms so there are matching transformers, where required, to make the match to 50 Ohms. Once again the use of the J310's are used in the remaining blocks. The BFO is a J310 and the RF Amp and IF Amp use the J310's in the cascode configuration to simulate a Dual Gate MOSET. The RF and IF amplifiers are virtually identical (and similar to the Product Detector). The differences are that the RF amplifier is designed for use at 7 - 7.3 MHz and the IF amp Stage design is centered on 12.1 MHz which is the IF design frequency. Later we will cover in detail the matching to the modules

Essentially the incoming frequency is up-converted to the IF frequency using the LO operating in the 5 MHz range. While an LCD would show 7 -7.3 MHz the actual LO RF being produced is in the 5 MHz range.

This is by design since there are those homebrewer's who do not like the use of modern technology such as the Arduino and AD9850. Some homebrewer's prefer an "absolute homebrew project" where everything and I mean everything must be scratch built from discrete components. There is much to be said for such an approach as it does lend itself to understanding what each and every component is contributing to the circuit and in detail learning how the circuit works. For myself I tend to build the first one with discrete components and then move on to the "black boxes". I know how the circuit works; but now want to take advantage of the expanded capabilities of the modern technologies. Both result in a working radio -- it is all a matter of choice.

Thus in an attempt to provide choice we will suggest that a VXO operating in/around 5 MHz as well as a conventional 5.0 MHz LC VFO or even a varactor tuned oscillator such as you might have in a self excited NE602 or SA612 will work. The choice is left to the builder. I will present a 5.0 MHz crystal switched VXO design that will give small segments of 40 Meters for those who feel technically or emotionally uncomfortable with the digital technology. Fortunately there are cheap computer crystals available to make this possible. The ranges cover portions of the SSB frequencies and only a limited amount of the CW frequency range. Mind you that is only because of the use of stock catalog crystals.

My plan is to build the Simpleceiver on a Bread Board just
like in the old days and my build philosophy is somewhat different than others.
A recent article I read on homebrewing started by the author suggesting that
the builder start with the most difficult circuit and get that working first.
Well I guess if you know in advance what will be the most difficult circuit
then I suppose that would work. But if you don't know which is the most
difficult, therefore some other process must be effected.

I always start at the back end of the project and work my
way forward which implicitly suggests that as you build forward the radio
itself becomes part of the test system. It also provides way points of proven
performance. Heathkit pioneered that method which I think is sound.

So I started with a Bread Board and installed the audio amp
on the board. When I built the Product Detector I left room on the board for
the BFO as it should be as a close as possible to the detector. The following
photos show the build and you can see the O Scope showing the waveform coming
from the BFO -- it is about 4 volts Peak to Peak. We may add a small trimmer
pot on the board to fine tune the BFO input to the Product Detector but that
can be done later. In passing note the frequency on the O Scope. That will be
used in the Arduino Code.

The very first bit and piece I would like to present is the BFO Circuit which is shown below. I did not simulate this in LT Spice but simply grabbed one of my stock circuits. The 50 PF trimmer cap is used to "wiggle" the 12.096 MHz crystal to place the signal on the appropriate frequency slope of the crystal filter. Since we are using the 5.0 MHz Frequency for the LO there is not a sideband inversion thus for LSB the BFO frequency must be ABOVE the filter center frequency in this case nominally 12.097500 MHz. Now this actual number becomes critical if you are using the Arduino + AD9850 as that then becomes the "Frequency Offset" used in the code. By having these numbers correct --when the display says 7.203 MHz -- it really is 7.203 MHz. This circuit gives a good account of itself and should be easy to replicate.

﻿

Now a few words about impedance interfaces where we attempt to make everything match 50 Ohms

Product Detector Output to Audio Amp. On the Output side we matched to 10K which essentially is a 10K Pot so we can match to the audio amplifier input. On the input side the Gate #1 likes to see about 2.2K (SSDRA ~ Hayward). But we want to have this at 50 Ohms. So we need a step up from 50 Ohms to 2.2 K --that match is a 1: 44 (2.2K/50). If we have a 3 turn primary to a 20 turn secondary --we are awful close. The impedance transformation is based on the turns ratio squared. Thus 3^2 = 9 and 20^2 = 400 and following 400/9 = 44.44 --close enough. Now you ask how did you know where to start --well that is Tribal Knowledge! I have wound enough matching transformers to just know what gets you close. BUT at one time I did create an excel spreadsheet and then I had a catalog of values. By the way get a large stock of FT-37-43 cores as they are used through out the build.

The 2nd IF Amp will feed the product detector and its output is already low impedance so a small 10 NF coupling capacitor can connect the output of the IF amp to the input of the Product Detector. On the input side it uses the same input as the Product Detectors so you have the same 3 Turn to 20 Turn Transformer.

The Crystal Filter was designed using a canned program on the internet. The Z in Z out is 300 Ohms. So we a match from 300 Ohms to 50 Ohms which is a 6 to 1 transformation. So if we used a 8 Turn to 19 Turn transformer we would be close. 8^2 = 64 and 19^2 = 381. Thus 381/64 = 5.953. Good Enough!!!! the same transformer would be used on the input side only reversed 50:300 in to 300:50 out.

The 1st IF amp is like the second only with a twist -- Hayward has suggested terminating a Post Mixer Amp with a resistive pad to provide a constant impedance to the amp and to prevent distortion. I simply used a 2dB Pi Resistive Pad Calculator on the Internet and came up with stock values of 470 Ohms and 12 Ohms. The In/Out is still 50 Ohms. So the output from the Pad is simply connected to the 50 Ohm Port on the crystal filter matching transformer. The input transformer is the same as the 2nd IF Amp and the Product Detector.

The SBL-1 is already at 50 Ohms In/Out and so Pins 3&4 are connected to the input of the 1st IF Amp.

The Receiver RF amp uses the same design as the IF amps only the frequency moved down to 7-7.3 MHz. The out is 50 Ohms and so a 10 NF between the output port of the amp and the RF in Port on the SBL-1 is all you need. The input side of the RF amp --yep just like the IF amps and the Product Detector is already set to 50 Ohms.

Thus it should become obvious we are using a standard design albeit with some slight modifications that is used through out the receiver. This same J310 DGM template will find itself in the transmitter stages. Thus a lot of design work is already done by simply creating the very first model.

Monday, November 16, 2015

Bits and Pieces Needed for the Transition from the Direct Conversion Receiver to the Super-heterodyne Receiver version of the Project.

Addendum: Oscilloscope plots of the BPF when driven by the Arduino + AD9850 and the filter connected to 50 Ohms. Amazingly what was theoretically shown in the simulation is in fact what is seen when connected to the real hardware. Let's hear it for SSDRA and LT Spice!

If you have followed along this far by building the Audio Amplifier stage, the J310 DGM Product Detector and the Arduino/AD9850 LO then we are ready to proceed with the next bits and pieces.

I struggled with having the prospective builder include the 40 Meter Band Pass Filter for the initial build and weighed carefully the balance between building yet another module or firing up the radio with a smoke test.

In my initial testing I saw that the radio worked with just these three modules thus proving the design concept. But the Direct Conversion Receiver should have a Band Pass Filter in the loop. Keep in mind with the antenna connected to Gate 1 --every bit of RF at all frequencies is being picked up by the antenna and sent to the J310's. As I have found out many times, builders are using just a chunk of wire laid out on the floor of the shack versus an installed antenna, up in the air and cut for the band in use.

We even had the individual who built the LBS Direct Conversion Receiver hook his receiver to the closest rain gutter for an anetenna and we got the usual "Your Receiver Isn't Working" email. If the builder intends to stop the project with just the direct conversion receiver, you still need to build the Band Pass Filter! YOU WILL ALSO NEED A PROPER ANTENNA!!!!!

So now we will take up next the Band Pass Filter (BPF) and its purpose in the circuit. Just as its name implies the BPF will pass certain bands of frequencies while rejecting others. These filters are used both on receive and especially on transmit. Not all filters are created equal as some have a very narrow band pass with steep signal rejections off of either side of the desired bandwidth, while others may be a broad as the proverbial barn door. Designing a filter that has the necessary bandwidth and covers the desired band while rejecting out of band signals can be done by hand!

W7ZOI in his Solid State Design for the Radio Amateur provides the necessary equations and detail to do this [located in the Appendix]. The same procedure is available in his EMRFD book and of course there is the Internet. I prefer to use the SSDRA in conjunction with the LT Spice Simulation. For this design I went into the SSDRA table of stock BPF constants and started with those values for the LT Spice simulation.

The simulation approach I find is most useful for fine tuning a filter and then almost in real time seeing the impact of the changed constants in the plot functionality. Another nice feature is that with LT Spice you can find a value of inductance that will work in the circuit while at the same time have a value that can be built with a specific number of turns. I haven't quite figured out how to wind an inductor on a toroid with 19.7568 turns --19 turns or 20 turns works fine business. But trust me that decimal value of inductance CAN be important.

Below is a simulation schematic of a 40 Meter Band Pass Filter that can be used with project. I call your attention to the two inductors whose value is 2.28 Uhy (Micro Henry). This value was tweaked so that with 20 Turns of #20 will fit on a Amidon T-68-2 (Red) Core and provide the needed inductance. A note here about values for the capacitors C3, 4 and C5 are standard capacitor values but an effort should be made to get close tolerance values (1% would be ideal but NPO or COG are a must!). The capacitors marked C1 and C2 at 164 PF is nothing more than a 150 PF (NPO or COG) in parallel with a 0-50 PF NPO Trimmer capacitor. Using this combination enables "fine tuning" the tank networks and by adjusting the trimmer "sneak up" on the required additional 14 PF.

Also noteworthy is that the SSDRA shows you how to treat "small values" of coupling capacitors. Some designs show a value of the center coupling capacitor like in the 1 PF range --it can be done with larger values of capacitor through a series/paralleling of the capacitors. For this build I actually had a stock of 7 PF caps but the bin was empty -- luckily long ago I purchased various values of small NPO trimmer caps [ 1-3 PF, 5 -13 PF, 3 -12 PF] where I used the 5 to 13 PF trimmer.

Important note: When using the simulation tool or measuring a built filter IT MUST BE TERMINATED WITH A 50 OHM NON-INDUCTIVE LOAD. In the simulations I used a 50 Ohm resistor in series with the source and the filter is terminated in 50 Ohms! Should a signal source be applied to the filter and measurements taken with a scope--it must be taken independent of being in the circuit across a terminated filter with a 50 Ohm termination!!!!

Below is a plot of this filter from what I call 10,000 feet over the spectrum 1 to 100 MHz. It has a narrow pass at our desired 40 Meter range. Note that any harmonics are about 40 dB down from this fundamental.

Now we have a more detailed view of our band pass window and the variation over most of the band is less than 1/2 dB. Being that this will be a SSB transceiver I was unconcerned that the lower 40 kHz was somewhat attenuated. But if the intent is to use the Simpleceiver on die hard low band edge work then using the simulation a bit of tweaking will reduce the attenuation on the lower 40 kHz. I will leave that exercise to the reader.﻿

So round up your parts and starting soldering. I usually place the Band Pass Filter ahead of the product detector in the Direct Conversion Receiver and if you happen to be using a DBM as a detector where it is a passive device (loss in the conversion process) then you might need an RF amplifier ahead of the detector. Thus my configuration for the is case would be the antenna > RF Amp> Band Pass Filter> Detector. But since our Product Detector is an active mixer the configuration would be Antenna > Band Pass Filter > Detector.

A photo of the prototype 40M Band Pass Filter -- it works!!!! I couldn't find my bag of 7 PF caps so I substituted a 5.5 to 13 PF trimmer cap in its place.

Addendum:

While I have said this Band Pass Filter works pretty good I ran some tests (we do have readers of this blog who believe nothing) so here are a bunch of screen shots of the response of the filter. For an RF Source I simply to the Arduino + AD9850 and ran that into one side and then on the other side I terminated the filter in 50 Ohms and the measured across the 50 Ohms with my DSO.

Our original simulation plot showed that we had a fairly flat response from about 7.040 MHz (who cares about low end CW) to about 7.280 MHz. Look on the scope face and you will see the frequencies that are being measured and sure enough our output plot follows that curve. Houston we have qa working Band Pass Filter!﻿

Saturday, November 14, 2015

The Arduino Pro-Mini and the AD9850

Now we will explore the Arduino Pro-Mini driving the AD9850 DDS Board to provide the Local Oscillator injection frequency for the Direct Conversion Receiver and later with a one line code change will provide the required injection frequency when the Simpleceiver is converted to a full fledged Super-heterodyne. The Intermediate Frequency (IF) was chosen to be 12.096 MHz largely in part because I had a stock of 12.096 MHz crystals. But there is a method to the madness because with a 5.0 MHz VXO or standard LC 5.0 MHz VFO you can also operate the Super-het on 40 Meters. (Keep in mind 7 +5 =12 where the LO is now below the incoming frequency. In practice high side mixing is better because of harmonic issues but will work nonetheless on the low side). A separate crystal oscillator operating at 12.096 MHz will provide the BFO injection frequency.

There is a lot of code floating around for driving the AD9850 with an Arduino; but one in particular I have found to be quite robust and that is the code available from Rich, AD7C. Here is a link to the code I used which is a modified version of AD7C's. Note my pin wiring (and sketch) is slightly different than what is shown in the original AD7C sketch. --he used pins 7,8,9 and 10. I used Pins 4, 5, 6 and 7. All important is that you do not change pins 2 and 3 which feed the encoder--these are the interrupt pins and they are cast in stone. [See my wiring diagram below.] Also note that my version of the sketch is in notepad so all you need to do is copy the sketch and drop that into an Arduino IDE. BUT depending upon which version of Arduino IDE you are using you may have to make other code modifications as some of the libraries such as you would use for the LCD are IDE dependent. What will work for an LCD display in an earlier version like Arduino 1.0.5 will not work in Arduino IDE 1.6.3 and higher.

Caution: Create a Sub-directory in your Arduino directory marked Simpleceiver and include the .ino sketch plus the rotary.h and rotary.cpp files. You must also have in the Arduino library folder the LiquidCrytsal_I2C libraries. If this is Greek to you then go back and start with square one on how to deploy the Arduino.

Not to worry! When you purchase an LCD display make certain you know its I2C address. The most common address is 0x27 but some are 0x3F and Adafruit uses A0. Now most of my code is written with IDE 1.0.5 but the most current IDE is 1.6.4. You will have to do some additional code changes if you are using a later IDE. There is a link on my website http://www.n6qw.com that tells you how to do that. Look under the listing N6QW Projects and it is the last item marked "LCD's and Arduino 1.6.3". Those software programmers sure make it tough for the ham homebrewers.

The AD9850 is an easy device to interface to the Arduino as there are but 4 connections in addition to +5VDC and ground. The Arduino has an on board regulator which supplies the 5 VDC for the AD9850 as well as 5 VDC for the display. My power source to the Arduino/AD9850 is a simple 9 Volt 1 Amp regulator (LM7809--TO-220 type) that connects to the 12 VDC rail.

The display is operated from the I2C protocol which comes with the Arduino architecture. Four wires are all that is needed and include the SDA (Data on Pin A4) and SCL (Clock on Pin A5) and the other two are + 5VDC and Ground. That said you do need what is called an I2C backpack which is a small interface board which takes those 4 lines and converts them to 16 lines to connect to the standard parallel LCD. In a sense the I2C backpack is a serial to parallel convertor.

Depending on whose Pro-Mini you purchase Pins A4 and A5 can be on one end of the board or on the top of the board. Try to avoid the top of the board version as getting wiring to the top of the board is not convenient. Now if this is the first time anyone has used a Pro-Mini you will also need to have USB to Serial convertor board so that you can interface and write code to the Pro-Mini. This is a one time purchase (about $6 USD) but can be reused for loading code on any Pro-Mini. Loading code on a Pro-Mini is a bit arcane. After connecting Pro-Mini to the convertor assembly and the USB end to the computer PRESS and Hold the reset button located on the Pro-Mini and then proceed to load the code. When a message appears at the bottom of the computer screen telling how large is the program quickly let go of the reset button and the code will load. The reason for the Pro-Mini --cheap!

Construction Notes

When I build a Digital Local Oscillator whether it is the AD9850 or Silicon Labs Si5351 I like to use a small piece of through hole prototype board. This type of board facilitates the use of pin headers and in line sockets. For interconnect wiring on the underside of the board I use #30 wire wrap wire and solder all connections. Typically these small prototype boards have board mounting holes in each corner which then enables using small aluminum type pillars to provide a means of elevating the boards so the wiring is not shorted to ground and facilitates mounting the board to the main chassis. The parts and pieces I typically use are shown below. I bought ten such prototype boards for $5 delivered from China thus quite a bargain. The pin headers and in line sockets I get from Jameco Electronics.

With this final piece covering the Arduino/AD9850 you are now ready to start building the Smpleceiver Direct Conversion Receiver.

Friday, November 13, 2015

More Investigation of the J310's configured as a Dual Gate MOSFET

Addendum #1: Decoding the Suffixes.

I am glad that every day I have a chance to learn something new and realize that being an "old dog" I can still learn some new tricks! A comment posted on Part 8 by Chris Horner made me realize that my old habits frequently trip me up.

Having started soldering my fingers together early in 1950, many electronic conventions have changed (all for the good) but sometimes they can trip us up. Take for instance how capacitor values were named. A 470 PF cap today was called a 470 micromicrofarfad or 470 MMF. A 100 Microfarad today designated as 100 Ufd, was written back then 100 MFd.. This now leads me to C1 and C7 and their designation on the simulation presented in Parts 8. The values written on the earlier schematic were 100 mF and 10 mf (harkening back to 1950 where I would have said 100 Microfarad or 10 Microfarad). But that is not how LT Spice really recognizes values where m = million. So thanks to Chris I once again have to wash my brain of the old habits.

His input caused me to look at what I had done and what would be done by those who build the J310 Product Detector. First let me say that I actually built the circuit with C7 using a 100 Microfarad electrolytic and C1 is a 10 Microfarad electrolytic. But this only adds more weight to the value of LT Spice so long as you enter the right values. Here is a short demonstration of why it is important to use the proper values in the circuit. Note C7 is shown as 100 mF and C1 is 10 mF on the earlier schematic.

So then I asked myself what really happens if one were to deliberately change the values of C1 in the simulation.

The first plot is with C1 being changed from a 10 Microfarad (or so I thought) to a 100 NF or 0.1 Microfarad. Wow the low frequency gain has really been impacted. The solid line shows that at the low end the gain is 12 dB less than what we were seeing previously and pretty much ignores the audio range.

The next plot is where C1 was changed to 10 Microfarad and we see the low end has now popped back up to what we had seen earlier. There is only about a 0.5 dB shift at the low end --so very acceptable.

Next we have the case where C1 is changed to 100 Microfarad and this now also aids in getting the higher frequency to cutoff so that the output is now pretty much in the audio range. This plot now looks like what was presented in Part 8.

Finally here is the revised schematic and I am now suggesting that C1 be changed to a 100 Microfarad electrolytic (10 is very acceptable but for the purists 100 is better). I now think I should be using a "u" instead of an "m" when designating large capacitors. I probably ought to read how to use LT Spice --guilty! I am a button pushers who doesn't read the instructions--but it does get me into trouble and frequently! Thanks Chris as your comment will be helpful to the greater ham homebrew community.

Stay Tuned -- the next post will be on the Arduino Pro-Mini and AD9850 used as the Local Oscillator. If you want to jump ahead we are using the code developed by AD7C. With the Arduino/AD9850 you will have all of the tools to build the Direct Conversion configuration of the Simpleceiver. Love that cool blue color! A simple code change later, will also enable the LO to be used with the Sinpleceiver when we install the crystal filter.

73's

Pete N6QW

Addendum #1

Thanks to one of the blog readers we have the decode that I had I read the LT Spice User's Guide first I wouldn't have goofed up! Then again I usually don't consult the roadmaps until I am really lost!

Wednesday, November 11, 2015

The Simpleceiver Direct Conversion Build is Working ~ Houston We Have Ignition!

We now have the J310 "Dual Gate MOSFET" Product Detector working as a 40M Direct Conversion Receiver! All of the upfront analysis we prepared has paid off in big dividends. The test configuration consists on the J310's as the Product Detector, an Arduino Pro-Mini driving an AD9850 in the range 7 to 7.3 MHz supplying the LO signal and the audio amplifier is the NE5534 driving an LM-380 . There is no additional amplification nor Band Pass Filter in line at this time. There is a short video to show "proof of life".

Our basic J310 "Dual Gate MOSFET" schematic was used as shown below. There are but two additions: 1) A 3 turn primary to 20 turn secondary wound on a FT-37-43 core (#26 wire) is used to match the 50 Ohms on the antenna connected to Gate 1 (J2), where the match is forced to 2.2K. C3 is not used with the matching transformer. This is a 44:1 match and 2) A 10 NF is connected to Gate 2 (J1) and the other end is connected to the AD9850 DDS.

This circuit forms the basic Dual Gate MOSFET template that will be used throughout the total Transreceiver project. With some slight changes this circuit will morph into a RF amplifier that can be used on either the receive or transmit side. More on this in a later post. By the Way in the above configuration this Product Detector is good for 18 dB gain. Run the LT Spice if you are not convinced or you could just look at Part 7.

Below is the schematic of the Audio Amplifier stage consisting of the NE5534 and the LM-380 --good for two watts. The LM-380 is used in many commercial transceivers and should allay the fears of those who pan the LM386. [Yes to the question-- this is a GIF --still don't understand why jpgs are no good.]

This is the test set up with Pro-Mini/AD9850, the Product Detector and the NE5534/LM380

The Arduino software I am using is based on the code from AD7C. The Microcontroller is the Arduino Pro-Min and for driving the AD9850 I am using pins 4,5,6 and 7. You must use Pins 2 & 3 for the encoder interrupt, Pin A3 is used for the step increment. The LCD is a 16X2 with a blue background.﻿

Back to the Simpleceiver Project

I must apologize for the diversion to work on the Ten Tec Model 150A. Before picking back up with the Simpleceiver I leave you with this comment: "If you can find a Model 150A --grab it!". Much of the work I did on the Model 150A will find its way into the Simpleceiver project especially the LT Spice simulation work with band pass and low pass filter networks. So while the anxious readers may be grumbling about the diversion --it will benefit you in the end.

Now back to the product detector circuit. We will use the circuit that was simulated earlier in LT Spice but convert this into actual hardware. The final step for the product detector is to mate it with the audio amplifier. The circuit below uses the 2N3819 JFETS but our final configuration is the J310. I do not have the spice simulation constants for the J310 but an A versus B comparison does show that the J310's are "hotter".

Once again the photo below is a GIF since I was given an input that jpg are not as good -- still baffles me why. I encourage the blog readers to download LT Spice and duplicate this circuit. I have added some notes about my experiments and it was most revealing.

Let us start with R4 which is 2.2 K Ohms. This was found by trial and error as entering new values with Spice is easier than doing a lot of hand calculations. A few trial runs will quickly tell "better or worse". What is evident that even 200 ohms or so can make a big difference. I tried 1800 and 2.5K. The value 2.2K seemed optimal.

This next takes us to the source resistor, R1 The spread values used went from 75 Ohms to 200 Ohms. The circuit I originally "lifted' from a publication had the value at 120 Ohms. Now it is obvious why!This experiment is a real lesson about 1) biasing impacts, 2) using specific values of resistance and 3) using 1% resistors. If one used a 20% resistor and cumulatively it was on the low side at or near 100 Ohms the stage gain is down by 6 dB. So this says much about the 1200 bargain resistors shipped from China for $5 --avoid them! The old adage I have a resistor but it is 100 Ohms not 120 Ohms what is the difference --well my friend the difference can be 6 dB!

Next was an evaluation of the impact of the output coupling connected to the Drain of J1. C4, L1 and C6 form a low pass filter to pass frequencies in the audio range. The product detector has two outputs as a result of the mixing process. One output is the difference with our expectation of a 1 KHz signal and the second is sum of the Local Oscillator and the incoming signal. In this case that would be about 14.06 MHz which we want to filter out of the following circuits. I used a 7dBm LO signal (1.414 Volts Peak to Peak) and the input signal at 0.3 microvolts (a typical weak signal). Capacitor C5 coupled the output to the R3 the 10K resistor which in real life is a volume control pot. C5 has impact on this circuit as well.

C4, L1 and C6 impact what is being passed out of the circuit for a fixed value of L1 at 1 milli-Henry (1000 micro-henry). If you make C4 and C4 larger (now at 0.001 Mfd.) this erodes the high frequency response of the network and there is a slight drop in gain in the audio range of 300 to 3000 Hz. C5 on the other hand if it is made larger will extend the lower frequency response well below the 300 Hz. Now these four components are interdependent meaning change one and while upper and lower ranges will shift the overall shape of the response curve may shift the center of the response curve. After several trials in the second photo is the resultant curve. This shows that the curve is fairly flat over the audio range and that the centering is at about 1 kHz. Try it yourself.

﻿

Now armed with the circuit values we can make out first run at building the product detector. Mind you we have simulated the circuit first and not soldered one wire or smoked one component. Keep in mind this is a perfect simulation! Don't get your underwear in a knot if you measure everything and get 15.1 dB gain versus 15.3 dB with a 120 Ohms resistor in the source. Now if you got only 5 dB gain then there is something wrong.

I want to take a moment to discuss construction practices and the lack of a specific troubleshooting process. An earlier published project called Let's Build Something (Kuo & Juliano QRP Quarterly Jan and April 2015) uses a modular approach very similar to the Simpleceiver. In fact it has the discrete audio amplifier stage. About a week ago Ben and I got the typical email "Your Circuit Doesn't Work!" Well we know it works because right now that circuit is tacked onto the Simpleceiver prototype and literally hundred have been built. Well long story short --there was a bad solder joint on one of the components and once fixed --we got an email "Your Circuit Works!" Well of course it does! But here is the point:

Lack of achieved performance has a root cause in the failure to review all of your work before applying power. Look for poor solder joints, wrong connections, no connections (same audio amp stage, different builder, same email message subject as the earlier one, with a photo in which we spotted a missing connection. Duh now it works), look for bad components, or wrong polarities on diodes and transistors, wrong value of components (10K instead of 100 Ohms). Going thru this type of process should be done first and also provides a template of check items if something still does not work. Powering up a circuit without doing a careful check first and finding it doesn't work -- the next step does not involve sending me an email with the subject line "Your circuit does not work".

The LT Spice sets the stage that the circuit does work and so now the problem is on your end! Having a documented trouble shooting process will help your resolve that issue. To that end the LT Spice enables you to look at the waveforms throughout the circuit --if you follow along with your build, it will become apparent the localized problem area. Use the tools that you have and the first tool is not sending me an email.

I hope to build the hardware in the next segment -- but I heartily encourage you to build the LT Spice model. Note I used 50 ohms series resistor with the input signals. By spending time with the model it will help you understand the circuit and how the individual components affect the overall performance. Oh the 2.2K Resistor R2 in the Gate 1 was chosen based on the Solid State Design Manual for the Radio Amateur. W7ZOI states that the Z in is in the range of 2 - 3K. In practice there will need to be matching from 50 Ohms to the 2.2K. This is a 1:44 match and can be done with a ferrite transformer FT-37-43 core with a 3 turn primary and a 20 turn secondary. 3^2 = 9 and 20^2 = 400. 400/9 = 44.4 which is close enough for government work.

Addendum:Below is a graph paper layout of how to build the product detector using the island squares.

Addendum #2:Our friend Nick, G8INE, located the expanded files for the JFETs which can be found on the LT Wiki. Here is the link http://ltwiki.org/?title=Components_Library_and_Circuits and specifically the link to A Large LTspice Folder from Bordodynov” . Now being somewhat skeptical about messing up a good thing when I downloaded the large file I only picked the sub files (libraries) for the JFETS. Next I went into my program files and replaced the original JFET library with the download. That worked really well and I now have significant increase in the number of JFET selections including the J310.

Nick also advised that his noodling with the actual J310 simulation indicated two resistor changes from the 2N3819. R1 is changed from 120 Ohms to 680 Ohms and R4 is changed from 2.2 K to 2.4 K. The device change and resistor changes increases the overall gain by 3 dB -- now we are smoking. Earlier I had mentioned that the J310 is a better device --now you know why! BTW don't forget to amend the parts placement sketch to include the changed values of R1 and r4

Addendum #3:Having the good fortune to own a $250K CNC machine (No the machine only cost $3K, but the $250K is what it cost to send my 3rd son to Mechanical Engineering school so he could learn how to build me a machine) I am able to rapidly crank out boards for my projects. In the photos below I started initially with the layout of just the Product Detector and then added island squares for the BFO. The second and third photos show the build and the location of the second set of squares for the BFO. Each squares is 2/10 of an inch on a side. Somewhat repeating myself --lay out the circuit using graph paper and then transfer the design to the PCB. Later today I hope to mate up the PD with the Audio Amp and LO to see if I can hear anything.